Aqueous latex coating compositions, such as latex paints, are used for a variety of applications, typically for the decoration or protection of surfaces. The latex coating compositions contain latex binders, typically emulsion polymers, maintained in liquid solvents. Upon application of the coating materials, the solvents evaporate and the binders harden into a mechanically rigid state while binding pigments, fillers, and other additives.
Several types of additives may be added to the latex coating compositions to improve their performance. For instance, latex compositions are often unstable at low temperature and when exposed to cycles of freezing and thawing. Ethylene glycol and propylene glycol are often used as anti-freeze additives to prevent the composition from freezing and to improve performance of the composition at low temperatures.
Additionally, coalescing solvents are often used as additives to lower the minimum film forming temperature (MFFT) of the composition, which allows the coating to be applied at lower temperature. Exemplary coalescing solvents include 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and 2-ethylhexyl benzoate. The coalescing solvents also affect the scrub resistance, i.e. durability, and blocking resistance, i.e. hardness, of the resultant coating.
Many of the additives used as coalescing agents and anti-freeze agents are effective for their purposes but are becoming less and less desirable because they are VOC's. After application of the latex composition to a substrate, the VOC's slowly evaporate into the surroundings. As permissible VOC levels continue to decrease as a result of increased environmental regulations, a need has developed in the art to produce latex compositions with lower VOC levels. However, there is also a need to maintain the performance of latex compositions even at these lower VOC levels.
Furthermore, the goal of achieving good coalescing properties, evidenced by good scrub resistance, has long been considered antagonistic to the goal of achieving good freeze-thaw stability, particularly in the absence of VOC solvent additives. Scrub resistance generally worsens as the MFFT of the latex increases and approaches the ambient curing temperature. Coalescing solvents, typically VOC's, have conventionally been added to lower the MFFT of coating compositions to improve the low temperature coalescence properties of the compositions. However, as the MFFT of an emulsion polymer is reduced, the freeze-thaw stability of the composition diminishes. Poor freeze-thaw resistance has traditionally been offset by the addition of ethylene or propylene glycol, each considered to be a VOC, which act as anti-freeze in the compositions. Thus, in the past, production of latex coating compositions having improved scrub resistance and freeze-thaw stability has required the addition of various VOC solvents.
Recent attempts to eliminate VOC's while maintaining acceptable coating performance have focused on modification of the latex binder to make it capable of coalescing in the absence of a coalescent additive. However, modification of the binder polymer tends to reduce at least one of low temperature coalescence, freeze-thaw resistance, or scrub resistance of the resulting coating compared to otherwise similar unmodified polymers applied with VOC additives.
It is, therefore, desired to produce an aqueous latex coating composition with reduced or no VOC content without reduction in coalescence, freeze-thaw stability, or scrub resistance, and without required modification of the latex binder.